Conventional single layer polypropylene homopolymer films are generally limited to applications requiring slick, stiff films with minimal mechanical properties and toughness and high sealing temperatures. The absence of limited cling and adhesive properties of such films makes the films unsuitable for use in stretch film and shrink film applications. The high sealing temperature, minimal properties and toughness make these films unsuitable for use in applications requiring sealable films with good mechanical properties, and toughness. Poor mechanical properties and sealability also prevents using such films in form/fill/seal applications such as commonly used for packaging.
Typical properties of monolayer polypropylene homopolymer films are low coefficients of friction; low tack; high modulus values; good machine direction tensile for oriented films; low stretch; low tear resistance; and low puncture resistance. Such films are conventionally used in applications such as confection wraps, textile packaging, tape (diapers and labels), stationary products, medical goods and sanitary goods.
A wide variety of multi-layer films incorporating a polypropylene layer have been developed to expand the suitability of polypropylene films for more diverse applications. Exemplary multi-layer films are disclosed in U.S. Pat. No. 6,500,563.
Polypropylene film layers generally seal at temperatures greater than 150° C., making such films useful in only a small number of sealing applications. The polypropylene resins are often blended with other polyolefins to provide monolayer films with good seal performance. Additionally, polypropylene film layers are often coextruded with other polyolefin layers to produce film structures with sealable skin layers.
Polypropylene film resins are restricted to extrusion operations not requiring melt strength, such as cast film applications and injection molding applications. Alternatively, polypropylene resins may be combined with other polyolefins to produce blends useful in extrusion applications requiring melt strength, such as blown film and blow molding applications.
The mechanical properties of cast polypropylene films may be modified by (i) increasing the polypropylene resin's molecular weight, (ii) incorporating a comonomer, such as ethylene, into the resin or (iii) blending a rubber such as an ethylene-propylene copolymer with the resin. Increasing the molecular weight of the polypropylene requires orienting the films more than for films produced from lower molecular weight resins in order to obtain good physical properties. Incorporating a comonomer into the resin results in impact copolymers with random copolymers having good impact and tear resistance, but at the expense of stiffness and strength. Producing blends incorporating rubbers is expensive because the rubbers must be compounded from bale-form into the polypropylene resin and the compounded material used to make film.
U.S. Pat. No. 6,500,563 discloses blending a propylene copolymer component incorporating from 5 wt. % to 40 wt. % of an alpha-olefin with an isotactic propylene polymer component to produce a polymeric material suitable for producing films. The blend may contain from 0 wt. % to 95 wt. %, preferably 2 wt. % to 40 wt. % of the crystalline isotactic polypropylene and from 5 wt. % to 100 wt. %, preferably 60 wt. % to 98 wt. % of a crystallizable ethylene and propylene copolymer. These films are described as elastic films having specific set properties.
As noted above, it is known to blend two or more polymeric materials to achieve beneficial performance characteristics in the resulting polymeric composition. One such conventional blending process is the blending of elastomeric and thermoplastic polymeric materials. For example, as described in U.S. Pat. No. 6,642,316, it is known to blend ethylene propylene based interpolymers with polypropylene so as to provide a polypropylene dispersed phase and an elastomeric continuous matrix phase. The overall blend may be elastomeric in nature. The ethylene propylene based interpolymers may be of the type described in WO 02/083754.
It is also known to blend an inverse distribution of phases, where the continuous matrix phase is a polypropylene and the dispersed phase is an ethylene propylene based interpolymer. The blends of this type are often referred to as TPO's (Thermoplastic Polyolefins). The dispersed phase acts as an impact modifier and improves the impact strength of the original polypropylene polymeric material.